Monday, December 17, 2007

Gene studies more tricky

I was quite enthusiastic about genetic studies, but I now realise that it is a bit more tricky than standard genetics.

There are two ways to look for genetic influences in a disorder. First, twin studies show the strength of the genetic component by comparing the probability of both twins having the disorder for the monozygotic (share the same DNA) and heterozygotic case. Second, you can look at affected families or populations to locate responsible genes in the chromosomes. The key is to associate a trait with the disorder. For example, blue eyes or red hair are good traits.

However, stuttering is more complicated. In stuttering it is not clear what the trait is. Stuttering is not really a trait as such, but symptoms, or I think what geneticists call a state. The real trait could be a dysfunctional motor region or the failure to initiate sounds properly. And, recovered stutterers might also have this trait but they do not stutter because they compensate well. And there might be many different traits leading to the same symptoms. Therefore, overt stuttering is only roughly related to the trait, and the signal strength of any research like twin studies or family trees will be significantly reduced.